Method for improving resolution limits of a stepper

ABSTRACT

A method for improving the resolution limits of a stepper is described. The region between the mask and the projection lens of the stepper is filled with a transparent material, wherein the reflection index of the transparent material is greater than one. Since the reflection index of the transparent material is greater than one, the diffracted angle of the diffracted light, formed after the parallel light from the light source passes through the mask, is reduced. A majority of the diffracted light can thus pass through the projection lens to improve the resolution limits of the stepper.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the priority benefit of Taiwanapplication serial no. 90125179, filed on Oct. 12, 2001.

BACKGROUNDING OF THE INVENTION

[0002] 1. Field of Invention

[0003] The present invention relates to a lithography process. Moreparticularly, the present invention relates to a method for improvingthe resolution limits of a stepper in a lithography process.

[0004] 2. Description of Related Art

[0005] The lithography process is an essential step in semiconductorprocessing, which includes the patterning of the various layers of thinfilm and the defining of the doped regions. Although a lithographyprocess is rather complicated, the theory behind lithography is simple.Typically, a photosensitive material is formed to cover the wafer. Afterpassing through a mask, the parallel light from a light source hits thephotosensitive material. The pattern on the mask is then completelytransferred to the photosensitive material on the wafer surface. Thisprocess of transferring a pattern on a mask to a photo-sensitivematerial through a parallel light of a light source is known asexposure.

[0006] A most common exposure technique is known as the projectionexposure, in which transferring the pattern on a mask is similar tousing a projector to project a pattern on a slide to a wallk. Sincethere is no direct contact between the mask and the wafer in accordanceto this exposure method, the pattern on the mask is prevented from beingdamaged. Moreover, the resolution of the transferred pattern is moredesirable. The projection exposure method has evolved to a newgeneration of the step-and-repeat method. The pattern on the mask usedfor this type of exposure method, however, is larger than thetransferred pattern. During the exposure process, the projected light,after passing through the mask, is reduced by an appropriate ratio andilluminates only a part of the wafer. The patterning of a wafer thus cannot complete in a single exposure process. To complete the exposure ofan entire wafer, the exposure process is repeated for multiple times toexpose the wafer step-by-step.

[0007]FIG. 1 illustrates the conventional projection exposure type ofstepper.

[0008] As shown in FIG. 1, the conventional stepper 120 includes a lightsource 100, a mask 102 and a projection lens 104, wherein the mask 102is placed between the light source 100 and the projection lens 104.

[0009] Using the stepper 120 to perform an exposure process issummarized as follow. After passing through a mask 102, a parallel light100 a from a light source 100 is diffracted at an angle. The diffractedlight 100 b, after passing through the projection lens 104, becomesprojected light and project on the wafer 106. Accordingly, the patternon the mask 102 is transferred to the wafer 106.

[0010] The region 108 between the mask 102 and the projection lens 104of the conventional stepper 102, however, is air. The diffracted angleof the diffracted light 100 b, which is formed after the parallel light100 a passes through the mask 102, is thereby too large. Therefore, thediffracted light 100 b, near the periphery of the projection lens 104,passes to the outside of the projection lens 104 and fails to project onthe wafer 106. As a result, the amount of light received by theprojection lens 104 is reduced and the exposure resolution is affected.

SUMMARY OF THE INVENTION

[0011] The present invention provides a method to improve the resolutionlimits of a stepper.

[0012] The present invention provides a method to improve the resolutionlimits of a stepper. After passing through the mask, the diffractionangle of the diffracted light is reduced to allow more diffracted lightto pass through the projection lens.

[0013] The present invention provides a method to improve the resolutionlimits of a stepper, wherein a majority of the diffracted light can passthrough the projection lens to improve the resolution limits of astepper.

[0014] The present invention provides a method to improve the resolutionlimits of a stepper, and the stepper includes a light source, a mask anda projection lens, wherein the mask is placed between the light sourceand the projection lens. A transparent material is then filled theregion between the lens and the mask. The reflective index of thetransparent material is greater than that of air (greater than 1),wherein the transparent material is, for example, air or a gel material.By filling the region between the mask and the projection lens with atransparent material, the diffraction angle of the diffracted light,formed after the parallel light passes through the mask, is reduced.More of the diffracted light thus passes through the projection lens toreduce the extent of the diffracted light to pass to the outside of theperiphery of the projection lens. The resolution limits of the stepperis thus improved to have a smaller line-width and to enhance themanufacturing capability.

[0015] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary, andare intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

[0017]FIG. 1 is an illustration of a conventional projection exposuretype of stepper; and

[0018]FIG. 2 illustrates the method for improving the resolution limitsof a stepper.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019]FIG. 2 illustrates the method for improving the resolution limitsof a stepper.

[0020] Referring to FIG. 2, a stepper 220 is provided, wherein thestepper 220 includes a light source 200, a mask 202 and a projectionlens 204. The light source 200 is formed with, for example, a mirror, amercury arc lamp, a filter and a focal lens. The projection lens 204 is,for example, a reduction projection system.

[0021] The mask 202 is placed between the light source 200 and theprojection lens 204. A parallel light 200 a is generated from a lightsource 200. Since after passing through the mask 202, the parallel light200 a would pass through the projection lens 204 to reach the surface ofthe wafer 206. The pattern from the mask 204 is then transferred to thewafer 206.

[0022] However, the diffracted angle of the diffracted light 200 b,formed after the parallel light passes through the mask 202, is toolarge. The diffracted light 200 b near the periphery of the projectionlens 204 therefore passes to the outside of the projection lens 104 andfails to project on the wafer 206.

[0023] Accordingly, the present invention provides a transparentmaterial to fill the region 208 between the mask 202 and the projectionlens 204. Since the reflection index of the transparent material isgreater than the reflection index of air, in other words, the reflectionindex of the transparent material is greater than one. According to thelaw of reflection:

n ₁ sin θ₁ =n ₂ sin θ₂

[0024] wherein,

[0025] n₁, n₁: the reflection index of the medium

[0026] θ₁: incident angle

[0027] θ₂: reflective angle

[0028] Based on the law of reflection, increasing the reflective indexof the medium reduces the reflective angle of the light. Filling theregion 208 between the mask 202 and the projection lens 204 with atransparent material with a reflective index greater than one thusreduce the diffraction angle of the diffracted light. The diffractedlight that passes to the outside of the peripheral of the projectionlens 204 is reduced to provide a better resolution limits of theexposure light.

[0029] The region 208 between the mask 202 and the projection lens 204is filled with a transparent material, such as a gas or a gel. Iffilling the region 208 with the transparent material with a reflectiveindex greater than one is a gas, the gas is, for example, argon. Iffilling the region 208 with the transparent material with a reflectiveindex greater than one is a gel, the gel is, for example, thegelatinized form of ZnSe, Al₂O₃, SiO₂, C₆H₆ CS₂ or CCl₄.

[0030] Since the aforementioned transparent materials with a reflectiveindex is greater than one, the diffraction angle of the diffracted light200 b is reduced, wherein the diffracted light 200 b is formed after theparallel light 200 a from a light source 200 passes through the mask202. Accordingly, a majority of the diffracted light 200 b near theperiphery of the projection lens 204 passes through the projection lens,reducing the chances of the diffracted light to pass to the outside ofthe projection lens. The resolution limits of the stepper is therebyimproved.

[0031] The present invention provides a method to improve the resolutionlimits of a stepper, wherein after passing through the mask, a majorityof the diffracted light passes through the projection lens. Byincreasing the amount of the diffracted light to pass through theprojection lens, the resolution limits of the stepper is thus improved.In other words, the line width can be reduced to enhance themanufacturing capability.

[0032] It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

What is claimed is:
 1. A method for improving resolution limits of astepper, comprising: providing a stepper, wherein the stepper includes alight source, a mask and a projection lens, wherein the mask is placedbetween the light source and the projection lens; and filling a regionbetween the mask and the projection lens with a transparent material,wherein a reflection index of the transparent material is greater thanthat of air.
 2. The method of claim 1, wherein the reflection index ofthe transparent material is greater than one.
 3. The method of claim 2,wherein the transparent material is a gas.
 4. The method of claim 3,wherein the gas includes argon.
 5. The method of claim 2, wherein thetransparent material includes a gel formed by gelatinizing a material.6. The method of claim 5, wherein the material is selected from thegroup consisting of ZnSe, Al₂O₃, SiO₂, C₆H₆ CS₂ or CCl₄.
 7. The methodof claim 1, wherein the light source is formed with a mirror, a mercuryarc lamp, a filter and a focal lens.
 8. The method of claim 1, whereinthe projection lens includes a reduction projection system.
 9. A methodto improve the resolution limits of a stepper by filling a regionbetween the mask and the projection lens of the stepper with a medium,wherein a reflective index of the medium is greater than that of air.10. The method of claim 9, wherein the reflective index of the medium isgreater than one.
 11. The method of claim 10, wherein the medium is agas.
 12. The method of claim 11, wherein the gas includes argon.
 13. Themethod of claim 10, wherein the medium includes a gel formed bygelatinized a material.
 14. The method of claim 13, wherein the materialis selected from the group consisting of ZnSe, Al₂O₃, SiO₂, C₆H₆ CS₂ orCCl₄.
 15. The method of claim 14, wherein the projection lens includes areduction projection system.